Cystic Fibrosis (CF) is the most common fatal genetic disease in humans (Boat, T. F. et al. in The Metabolic Basis of Inherited Diseases (Scriver, C. R. et al. eds., McGraw-Hill, N.Y. (1989)). Approximately one in every 2,500 infants in the United States is born with the disease. At the present time, there are approximately 30,000 CF patients in the United States. Despite current standard therapy, the median age of survival is only 26 years. Disease of the pulmonary airways is the major cause of morbidity and is responsible for 95% of the mortality. The first manifestation of lung disease is often a cough, followed by progressive dyspnea. Tenacious sputum becomes purulent because of colonization of Staphylococcus and then with Pseudomonas. Chronic bronchitis and bronchiectasis can be partially treated with current therapy, but the course is punctuated by increasingly frequent exacerbations of the pulmonary disease. As the disease progresses, the patient's activity is progressively limited. End-stage lung disease is heralded by increasing hypoxemia, pulmonary hypertension, and cor pulnonale.
The upper airways of the nose and sinuses are also involved by CF. Most patients with CF develop chronic sinusitis. Nasal polyps occur in 15–20% of patients and are common by the second decade of life. Gastrointestinal problems are also frequent in CF; infants may suffer meconium ileus. Exocrine pancreatic insufficiency, which produces symptoms of malabsorption, is present in the large majority of patients with CF. Males are almost uniformly infertile and fertility is decreased in females.
Based on both genetic and molecular analyses, a gene associated with CF was isolated as part of 21 individual cDNA clones and its protein product predicted (Kerem, B. S. et al. (1989) Science 245:1073–1080; Riordan, J. R. et al. (1989) Science 245:1066–1073; Rommens, J. M. et al. (1989) Science 245:1059–1065)). U.S. Ser. No. 07/488,307 describes the construction of the gene into a continuous strand, expression of the gene as a functional protein and confirmation that mutations of the gene are responsible for CF. (See also Gregory, R. J. et al. (1990) Nature 347:382–386; Rich, D. P. et al. (1990) Nature 347:358–362). The co-pending patent application also discloses experiments which show that proteins expressed from wild type but not a mutant version of the cDNA complemented the defect in the cAMP regulated chloride channel shown previously to be characteristic of CF.
The protein product of the CF associated gene is called the cystic fibrosis transmembrane conductance regulator (CFTR) (Riordan, J. R. et al. (1989) Science 245:1066–1073). CFTR is a protein of approximately 1480 amino acids made up of two repeated elements, each comprising six transmembrane segments and a nucleotide binding domain. The two repeats are separated by a large, polar, so-called R-domain containing multiple potential phosphorylation sites. Based on its predicted domain structure, CFTR is a member of a class of related proteins which includes the multi-drug resistance (MDR) or P-glycoprotein, bovine adenyl cyclase, the yeast STE6 protein as well as several bacterial amino acid transport proteins (Riordan, J. R. et al. (1989) Science 245:1066–1073; Hyde, S. C. et al. (1990) Nature 346:362–365). Proteins in this group, characteristically, are involved in pumping molecules into or out of cells.
CFTR has been postulated to regulate the outward flow of anions from epithelial cells in response to phosphorylation by cyclic AMP-dependent protein kinase or protein kinase C (Riordan, J. R. et al. (1989) Science 245:1066–1073; Welsh, 1986; Frizzell, R. A. et al. (1986) Science 233:558–560; Welsh, M. J. and Liedtke, C. M. (1986) Nature 322:467; Li, M. et al. (1988) Nature 331:358–360; Hwang, T-C. et al. (1989) Science 244:1351–1353).
Sequence analysis of the CFTR gene of CF chromosomes has revealed a variety of mutations (Cutting, G. R. et al. (1990) Nature 346:366–369; Dean, M. et al. (1990) Cell 61:863–870; and Kerem, B-S. et al. (1989) Science 245:1073–1080; Kerem, B-S. et al. (1990) Proc. Natl. Acad, Sci. USA 87:8447–8451). Population studies have indicated that the most common CF mutation, a deletion of the 3 nucleotides that encode phenylalanine at position 508 of the CFTR amino acid sequence (AF508), is associated with approximately 70% of the cases of cystic fibrosis. This mutation results in the failure of an epithelial cell chloride channel to respond to cAMP (Frizzell R. A. et al. (1986) Science 233:558–560; Welsh, M. J. (1986) Science 232:1648–1650.; Li, M. et al. (1988) Nature 331:358–360; Quinton, P. M. (1989) Clin. Chem. 35:726–730). In airway cells, this leads to an imbalance in ion and fluid transport. It is widely believed that this causes abnormal mucus secretion, and ultimately results in pulmonary infection and epithelial cell damage.
Studies on the biosynthesis (Cheng, S. H. et al. (1990) Cell 63:827–834; Gregory, R. J. et al. (1991) Mol. Cell Biol. 11:3886–3893) and localization (Denning, G. M. et al. (1992) J. Cell Biol. 118:551–559 ) of CFTR AF508, as well as other CFTR mutants, indicate that many CFTR mutant proteins are not processed correctly and, as a result, are not delivered to the plasma membrane (Gregory, R. J. et al. (1991) Mol. Cell Biol. 11:3886–3893). These conclusions are consistent with earlier functional studies which failed to detect cAMP-stimulated C1.sup.-channels in cells expressing CFTR AF508 (Rich, D. P. et al. (1990) Nature 347:358–363; Anderson, M. P. et al. (1991) Science 251:679–682).
To date, the primary objectives of treatment for CF have been to control infection, promote mucus clearance, and improve nutrition (Boat, T. F. et al. in The Metabolic Basis of Inherited Diseases (Scriver, C. R. et al. eds., McGraw-Hill, New York (1989)). Intensive antibiotic use and a program of postural drainage with chest percussion are the mainstays of therapy. However, as the disease progresses, frequent hospitalizations are required. Nutritional regimens include pancreatic enzymes and fat-soluble vitamins. Bronchodilators are used at times. Corticosteroids have been used to reduce inflammation, but they may produce significant adverse effects and their benefits are not certain. In extreme cases, lung transplantation is sometimes attempted (Marshall, S. et al. (1990) Chest 98:1488).
Most efforts to develop new therapies for CF have focused on the pulmonary complications. Because CF mucus consists of a high concentration of DNA, derived from lysed neutrophils, one approach has been to develop recombinant human DNase (Shak, S. et al. (1990) Proc. Natl. Sci. Acad USA 87:9188). Preliminary reports suggest that aerosolized enzyme may be effective in reducing the viscosity of mucus. This could be helpful in clearing the airways of obstruction and perhaps in reducing infections. In an attempt to limit damage caused by anexcess of neutrophil derived elastase, protease inhibitors have been tested. For example, alpha-1-antitrypsin purified from human plasma has been aerosolized to deliver enzyme activity to lungs of CF patients (McElvaney, N. et al. (1991) The Lancet 337:392). Another approach would be the use of agents to inhibit the action of oxidants derived from neutrophils. Although biochemical parameters have been successfully measured, the long term beneficial effects of these treatments have not been established.
Using a different rationale, other investigators have attempted to use pharmacological agents to reverse the abnormally decreased chloride secretion and increased sodium absorption in CF airways. Defective electrolyte transport by airway epithelial is thought to alter the composition of the respiratory secretions and mucus (Boat, T. F. et al. in The Metabolic Basis of Inherited Diseases (Scriver, C. R. et al. eds., McGraw-Hill, New York (1989); Quinton, P. M. (1990) FASEB J 4:2709–2717). Hence, pharmacological treatments aimed at correcting the abnormalities in electrolyte transport could be beneficial. Trials are in progress with aerosolized versions of the drug amiloride; amiloride is a diuretic that inhibits sodium channels, thereby inhibiting sodium absorption Initial results indicate that the drug, is safe and suggest a slight change in the rate of disease progression, as measured by lung function tests (Knowles, M. et al. (1990) N. Eng. J Med. 322:1189–1194; App, E. (1990) Am. Rev. Respir. Dis. 141:605. Nucleotides, such as ATP or UTP, stimulate purinergic receptors in the airway epithelium. As a result, they open a class of chloride channel that is different from CFTR chloride channels. In vitro studies indicate that ATP and UTP can stimulate chloride secretion (Knowles. M. et al. (1991) N. Eng. J. Med. 325:533). Preliminary trails to test the ability of nucleotides to stimulate secretion in vivo, and thereby correct the electrolyte transport abnormalities are underway.
Despite progress in therapy, cystic fibrosis remains a lethal disease, and no current therapy treats the basic defect. However, two general approaches may prove feasible. These are: 1) protein replacement therapy to deliver the wild type protein to patients to augment their defective protein, and; 2) gene replacement therapy to deliver wild type copies of the CF associated gene. Since the most life threatening manifestations of CF involve pulmonary complications, epithelial cells of the upper airways are appropriate target cells for therapy.
The feasibility of gene therapy has been established by introducing a wild type cDNA into epithelial cells from a CF patient and demonstrating complementation of the hallmark defect in chloride ion transport (Rich, D. P. et al. (1990) Nature 347:358–363). This initial work involved cells in tissue culture, however, subsequent work has shown that to deliver the gene to the airways of whole animals, defective adenoviruses may be useful (Rosenfeld, (1992) Cell 68:143–155. However, the safety and effectiveness of using defective adenoviruses remain to be demonstrated.